Zona pellucida- the clear glycoprotein layer between the corona radiate and the oocyte’s membrane

One of the glycoproteins in the zona pellucida called ZP 3 acts a sperm receptor. Its binding to specific membrane proteins in the sperm head and triggers the acrosmomal reaction, the release of the contents of the acrosome. The acrosomal enzymes digest a path through the zona pellucid as the lashing sperm pushes the sperm cell onward.

fast block to polyspermy

The fusion of a sperm cell with a secondary oocyte sets in motion events that block polyspermy, fertilization by more than one sperm cells. Within few second, the cell membrane of the oocyte depolarizes, which act as fast block to polyspermy-a depolarized oocyte cannot fuse with another sperm. Depolarization triggers the intracellular release of Ca+2. The molecules released by exocytosis inactivate ZP3 and harden the entire zona pellucid, events called the slow block to polyspermy

Cleavage of the zygote

After fertilization, rapid mitotic cell division of the zygote called cleavage take place.

First clevage

The 1st division of the zygote begins about 24 hours after fertilization and is completed about 6 hours; 1st cleavage in 30 hours produces 2 blastomeres

Second cleavage

By the second day after fertilization, the second cleavage is completed and there are 4 cells

morula

By the end of the 3rd day there are 16 cells. By day 4 has formed a solid ball of cells called a morula. When the morula enters the uterine cavity on day 4th a glycogen-rich secretion from the glands of the endometrium of the uterus passes into the urine cavity and enters the morula through the zona pellucid. This fluid called uterine milk. It provides nourishment for the developing morula.

Blastocys

At the 32 cells stage, the fluid enters the morula, collects between the blastomeres, and reorganizes them around a large fluid-filled cavity called the blastocyst cavity, also called the blastocoels. Once blastocyst cavity is formed, the developing mass is called the blastocyst. A blastocyst is a hollow ball of cells that enters the uterine cavity by day 5

Formation of the blastocyst

During the formation of the blastocyst two distinct cell population arise:Embryoblast and Trophoblast

Embryoblast and Trophoblast

Embryoblast, or inner cell mass is located internally and eventually develops into the embryo.

Trophoblast is the outer superficial layer of cells. It will develop into the outer chorionic sac that surrounds the fetus and the fetal portion of the placenta, the site of exchange of nutrients and wastes between the mother and fetus.

Fraternal twins (dizygotic)

Independent release of 2 oocytes fertilized by 2 separate sperm. They are the same age, but genetically they are dissimilar, may me or not be the same sex

Identical twins (monozygotic)

2 individuals that develop from a single fertilized ovum, genetically identical & always the same sex, if ovum does not completely separate, conjoined twins (share some body structures).

male pronucleus

Once inside the oocyte, the sperm loses its tail & becomes a male pronucleus.

Implantation (7 days after fertilization)

The blastocytes remains free within the uterine cavity for about 2 days before it attaches to the uterine. About 6 days after fertilization, the blastocyst attaches to the endometrium in a process called implantation. Blastocyst orients with the inner cell mass toward the endometrium. About 7 days after fertilization, the blastocyst attaches to the endometrium more firmly, and endometrium becomes more vascularized and glands enlarge.

Second week of development (8th days after fertilization)

About 8 days after fertilization, the trophoblast develops into two layers in the region of contact between the blastocyst and endometrium. During implantation the syncytiotrophoblast secretes enzymes that enable the blastocyst to penetrate the uterine line by digesting the endometrial cells. Cytotrophoblast is distinct layer of cells that defines the original shape of the embryo.

chorionic gonadotropin(hCG).

Another secretion of trophoblast is human chorionic gonadotropin(hCG). Human chorionic gonadotropin rescues the corpus luteum from degeneration and sustains (continue) its secretion of progesterone and estrogen. These hormones maintain the uterine lining in a secretory state, preventing menstruation. Peak secretion of hCG occurs about the 9th week of pregnancy at which time the placenta is fully developed and produces the progesterone and estrogen that continue to sustain the pregnancy. The presence of hCG in maternal blood or urine is an indicator of pregnancy.

Ectopic Pregnancy

Ectopic Pregnancy is a development of an embryo outside the uterus. Common causes are blockages of uterine tube such as tumors or scars from pelvic inflammatory disease. Symptoms are missed menstrual cycles, bleeding & acute pain.Twice as common in smokers because nicotine paralyzes the cilia.

Development of the Bilaminar Embryonic Disc

Cells of the embryoblast also differentiate into two layers around 8 days after fertilization: hypoblast (primitive endoderm) epiblast (primitive ectoderm). Cells of the hypoblast and epiblast together form a flat disc referred to as the bilaminar embryonic disc. Soon, a small cavity appears within the epiblast and eventually enlarges to form the amniotic cavity. As the amniotic cavity enlarges, a single layer of squamous cells forms a dome-like roof above the epiblast cells called the amnion.

Also on the eight day after fertilization, cells at the edge of the hypoblast migrate and cover the inner surface of the blastocyst wall. The migrating columnar cells become squamous (flat) and then form a thin membrane referred to as the exocoelomic membrane. Together with the hypoblast, the exocoelomic membrane forms the wall of the yolk sac. As result, the bilaminar embryonic disc is now positioned between the amniotic cavity and yolk sac.

Function of Yolk Sac

– supplies nutrient to the embryo during the second and third weeks of development

– is the source of blood cells from the 3rd through 6th weeks

– contains the 1st cells wthat will eventually migrate into developing gonads, differentiate into the primitive germ cells, and form gamets

– forms part pf the gut (gastrointestinal tract)

– function as a shock absorber

– helps prevent drying out of the embryo

Development of Sinusoids

On the 9th day after fertilization, the blastocyst becomes completely embedded in the endometrium. As the syncytiotrophoblast expands, small spaces called lacunae. By the 12th day of development, interconnecting spaces called lacunar networks. Endometrial capillaries around the developing embryo become dilated and are referred to as maternal sinusoids.

Development of the Extraembryonic Coelom

About the 12 th day after fertilization, the extraembryonic mesoderm develops. These mesoderm cells are derived from the yolk sac and form connective tissue layer around the amnion and yolk sac.

Development of the chorion

The extraembryonic mesoderm together with two layers of the trophoblast (the cytotrophoblast and syncytiotrophoblast) form the chorion. The chorion surrounds the embryo and later the fetus. The chorion also protects the embryo and fetus from the immune responses of the mother in two ways:

– it secrets proteins that blocks antibody production by the mother

– it promotes the production of T lymphocytes that suppress the normal immune response in the uterus

By the end of the second week of development, the bilaminar embryonic disc becomes connected to the trophoblast by a band of extraembryonic mesoderm called the connecting stalk. The connecting stalk is future umbilical cord.

Placenta

Placenta forms during 3rd month. By the beginning of the 12th week, the placenta has two distinct parts: The fetal portion formed by the chorionic villi of the chorion. The maternal portion formed by the deciduas basalis of the endometrioum

The actual connection between the placenta and embryo, and later the fetus, is through the umbilical cord, which develops from the connecting stalk and is usually about 2 cm (1in) wide and about 24 in in length. The ubilical cord contains:

· 2 arteries that carry deoxygenated blood to the placenta

· 1 umbilical vein that carries oxygenated blood to the fetus

Afterbirth

After the birth of the baby, the placenta detaches from the uterus and is therefore termed the afterbirth.

Umbilicus

The small portion (about 1 in) of the cord that remains attached to the infant begins to wither and falls off, usually within 12-15 days after birth. The area where the cord was attached become covered by a thin layer of skin, and scar tissue forms that is called umbilicus (navel).

Placenta Previa

Placenta is implanted near or covering os of cervix. Occurs in 1 to 250 live births. May lead to spontaneous abortion, premature birth or increased maternal mortality. Major symptom is sudden, painless bright red vaginal bleeding in the 3rd trimester. Cesarean section is preferred delivery method

Relaxin a hormone produces first by the corpus luteum of the ovary and later by the placenta, increase the flexibility of the pubic symphysis and ligaments of the sacroiliac and sacrococcygeal joints and helps dilate the uterine cervix during labor.

Corticotropin-releasing hormone (CRH). CRH is thought to be a part of the “clock” that establish the timing of birth. Secretion of CRH by the placenta begins at about 12 weeks and increase enormously toward the end of pregnancy. CRH from the placenta has a second important effect: it increase secretion of cortisol, which is needed for maturation of the fetal lungs and the production of surfactant.

Early Pregnancy Tests

Detect human chorionic gonadotropin (hCG) in the urine as soon as 8 days after fertilization

As early as 11 weeks. Results in few days. Chance of spontaneous abortion 1-2%. 30 mg of placenta removed by suction through cervix or with needle through abdomen. Chromosomal analysis reveals same results as amniocentesis

Amniocentesis

Involves withdrawing some of the amniotic fluid that bathes the developing fetus and analyzing the fetal cells and dissolved substances. It used to test for the presence of certain genetic disorder, such as Down syndrome, hemophilia, sickle cell disease.Test at 14-16 weeks. Results back in one month. Needle through abdominal wall & uterus. Chance of spontaneous abortion is 0.5%

Labor begins when progesterone is overcome by an increase in the levels of estrogen.Progesterone inhibits uterine contraction. The rise in estrogen results from increasing secretion by the placenta of corticotrophin-releasing hormone, which stimulates the anterior pituitary gland of the fetus to secrete ACTH (adrenocorticotropic hormone); in turn ACTH stimulates the fetal adrenal gland to secrete cortisol and dehydroepiandrosterone (DHEA), the major adrenal androgen. The placenta then converts DHEA to estrogen. Estrogen overcomes progesterone and labor begins. High levels of estrogen cause the number of receptors for oxytocin on uterine muscle fibers to increase, and cause uterine muscle fibers to form gap junction with one another. Oxytocin released by the posterior pituitary stimulates uterine contraction, and relaxin from the placenta assist by increasing the flexibility of the pubic symphysis and helping dilate the uterine cervix.

Premature Infants is any baby weighs less than 5lb. 8oz at birth. Causes-poor prenatal care, drug abuse, placenta previa, preeclampsia, eclampsia, young or old mother (below 16 or above 35)

Lactation

Lactation is the secretion and ejection of milk from the mammary glands. Prolactin is a hormone from anterior pituitary promotes milk synthesis, increases during pregnancy, but progesterone inhibits effects of prolactin until after delivery. After delivery, progesterone levels drop & suckling increases the release of prolactin & oxytocin (milk ejection reflex). Oxytocin released from posterior pituitary. Carried by bloodstream to the mammary gland, oxytocin stimulates contraction of myoepithelial cells surrounding the glandular cells and ducts. The resulting compression moves the milk from the alveoli of the mammary glands into the mammary ducts, where it can be suckled. Colostrum = cloudy fluid released for few days. True milk produced by 4th day.

Infertility

Infertility. Someone is unable to have a baby.

Female. 10% of reproductive age U.S. population, ovarian disease or obstruction of uterine tubes, inadequate or excessive body fat

In Vitro Fertilization is commonly referred to as IVF. IVF is the process of fertilization by manually combining an egg and sperm in a laboratory dish. When the IVF procedure is successful, the process is combined with a procedure known as embryo transfer, which is used to physically place the embryo in the uterus.

Gamete intrafallopian transfer

Gamete intrafallopian transfer (GIFT) is an assisted reproductive procedure which involves removing a woman’s eggs, mixing them with sperm and immediately placing them into your fallopian tube